This invention relates to the polymerization of conjugated diene monomers, and to the polymerization of such monomers to polymers having increased amounts of 1,2-structure. More particularly, the invention relates to a catalyst system and the polymerization of conjugated dienes in the presence of such catalyst system which comprises an anionic initiator based on lithium, and at least one cyclic acetal of a glyoxal as a modifier.
The polymerization of conjugated dienes, and particularly of 1,3-dienes to useful rubbery polymers with various initiator systems is known. Such polymerizations can be initiated with organometallic compounds wherein the metal is a Group I metal such as lithium. For example, 1,3-dienes such as butadienes have been polymerized with butyl lithium in hydrocarbon diluents. The polymers prepared by this process generally contain large amounts of 1,4-structure and minor amounts of 1,2-structure. For many applications, it is desirable to increase the ratio of 1,2-structure in the polymers because this increases the cure rate in free radical cure systems. Such polymers and copolymers containing increased amounts of 1,2-structure are useful for tire rubbers, molded rubber goods, molding compounds, surface coatings, etc.
The use of mixtures of organometallic compounds for initiating polymerization of conjugated dienes has been suggested in several patents. For example, U.S. Pat. No. 3,646,321 describes the polymerization of conjugated dienes utilizing hydrocarbon-soluble organometallic complexes of metals of Group I and II(a) of the Periodic System. The complexes are exemplified by the complexes of di-n-butyl magnesium with n-butyl-lithium or n-butyl potassium. U.S. Pat. No. 3,847,883 also describes the use of complexes of diorganomagnesium compounds with various organometallic compounds of the metals of Group I.
U.S. Pat. No. 3,817,955 discloses the polymerization of conjugated dienes such as butadiene with a catalyst complex which comprises a dialkyl magnesium with either an alkali metal hydride or a tetraalkylammonium hydride. However, only low molecular weight polymers are reported. The use of a catalyst system comprising dibutyl magnesium and a barium dialkoxide such as barium di-tertiary butoxide is described in U.S. Pat. No. 3,846,385.
There also has been described in the prior art other catalyst or initiator systems which include organometallic compounds of the type described above which are modified by the inclusion of other materials. For example, U.S. Pat. No. 3,451,988 describes a polymerization catalyst for polymerizing vinyl compounds that utilizes mixtures of hydrocarbyl lithium and magnesium compounds, and a modifier which is a bi-functional Lewis base such as tertiary diamines including tetramethylethylene diamine.
U.S. Pat. No. 3,207,742 describes a process for preparing rubbery polymers of 1,3-dienes having an increased ratio of 1,2-structure. Such polymers are obtained by conducting a polymerization of a 1,3-diene in the presence of a lithium hydrocarbon catalyst, and a hexamethylphosphoramide modifier. The amount of the phosphoramide present is from about 10 to 0.1 mole percent based on the amount of diene present.